Telephone system



June 9, 1942. M. A. LOGAN TELEPHONE SYSTEM Filed March 30, 1940 2 Sheets-Sheet 1 INVENTOR v M. 'A. LOGAN WWMHQ ATTORNEY "June 9, 1942. M. A. LOGAN 2,285,524

' TELEPHONE SYSTEM 7 Filed March 30, 1940 ZSheet-Sheet 2 5% WM L 92 E E N m2 Patented June. 9, 1942 S'ETEJS Ah TELEPHGN E SYSTEM Application March 30, 1940, Serial No. 326,857

14 Claims.

This invention relates to telephone systems and particularly to the signaling, controlling, and switching facilities used in these systems.

The objects of the invention are to improve the signaling and controlling operations performed over telephone lines, such as subscribers lines; to insure the proper operation of the signaling and controlling facilities despite the presence of line leakage or other adverse conditions; to increase the effectiveness of the means for testing and making connection with these lines; and to improve these systems in other respects.

Suggestions have been made in the past to improve the individual circuit associated with the subscribers line by substituting a discharge tube for the usual line relay. In some cases these tubes and the circuits provided therewith have been arranged to eliminate both the line and cut-off relays ordinarily used in practice. It has been found, however, that these tube line circuits are frequently subject to false operation in the presence of excessive line leakage.

According to the present invention the advantages of using discharge tubes for performing the functions of a subscribers line circuit are retained, and at the same time the disadvantage above noted is overcome. These results are obtained by means of a line circuit in which the control gap of a discharge tube ionizes in response to a voltage produced by the line closure at the substation and fires the main gap of the tube, a relay in the main gap circuit serving to start a line finder in operation; and in which two sources of alternating potential of the same phase and frequency are connected respectively in the cathode and anode branches of the main gap circuit. The cumulative voltage of these two sources acts to quench the main gap discharge on each alternate half cycle, While the source in the cathode branch serves to oppose the voltage produced across the control gap by line leakage. This arrangement insures that the tube is quenched as soon as the control gap ionizing voltage is removed, since the voltage across the control gap resulting from leakage n the line, which might otherwise be sufiicient to sustain the control gap ionization, is opposed simultaneously with the quenching of the main gap. Hence on the next opposing half cycle following the removal of the ionizing voltage from the control gap both gaps are quenched simultaneously, and the tube remains deionized. This prevents false operation, which might otherwise occur where line leakage currents are excessive.

A feature of the invention is a line-finder testflow through both of these differential windings.

However, when the calling line is reached, only one of the tubes discharges, whereupon the corresponding windin of the test relay is energized and the relay operates to stop the hunting action of the finder switch.

These and other features of the invention will be described in detail in the following specification.

In the drawings accompanying the specification:

Figs. 1 and 2 disclose a portion of an auto matic telephone system incorporating one form of the present invention. Fig. 1 shows the details of a calling subscribers line and line circuit and a part of a line-finder switch, and Fig. 2 illustrates the remainder of the finder circuit, and also selector and connector switches partly in diagrammatic form and partly in detail. Fig. 3 is a modification of the testing circuit of the line finder.

The advantages of using discharge tubes to perform the functions of line relays in subscribers line circuits have been explained above. There are, however, certain conditions under which these tubes may be exposed to false operation, particularly if the line is subject to leakage. In order that the tube may be ready to respond to the closure of the subscribers loop when a call is made, the control electrode and the central ofiice battery are connected in series with resistances through normal contacts of the cut-off relay to the subscribers line. If the leakage resistance of the line is relatively low, substantial leakage currents flow from the central ofiice battery through the coupling resistance, producing a voltage across the control gap of the tube. This voltage, although too small to ionize the control gap of the tube, may be sufiicient when added to the normal biasing voltage of the control gap to sustain the ionization once it has started. The conditions under which this leakage voltage would, unless some provision were made to the contrary, be sufficient to cause false operation will now be explained.

As soon as the line finder has found the line in response to the calling condition thereon, the cut-01f relay operates in the usual manner and disconnects the line tube, opening the control gap circuit and permitting the quenching of the anode gap to release the start relay connected in circuit therewith. When the called subscriber answers, the usual current reversal is made, and the capacity of the calling subscribers line is charged in the opposite direction from the charge it receives from the central ofiice battery connected thereto through the normal contacts of the cutoff relay when the line is idle. When, therefore, the connection is released and the cut-off relay closes its back contacts to reconnect the central ofiice battery to the line, a relatively heavy charging current fiows through the coupling resistance to discharge the line in one direction and recharge it in the opposite direction. This discharge current creates a voltage across the control gap which may be sufiicient to ionize it, thereby causing current to flow in the anode circuit and through the group start relay. If at this time the leakage voltage on the line is sufficient to raise the total voltage across the control gap above the sustaining value, the tube will remain ionized after the line charging current has ceased to flow, and the group relay will fully operate and cause the false starting of a line finder.

Another condition under which false operation may occur exists where the line finder is designed to test in its hunting operation for a marginal potential on the sleeve conductor of the line, the presence of which depends upon the flow of current over the subscribers closed loop. With an arrangement of this kind should the subscriber replace his receiver after the finder has started in operation but before it reaches the calling line, the potential identifying the calling line is removed and is replaced by the usual idle line potential. Therefore, the finder fails to stop on the line that initiated the call, and the cut-off relay is not operated to open the control gap of the tube, which remains ionized because of the excessive leakage voltage. The result is that the main anode gap continues its ionization and the group start relay falsely indicates a calling condition.

In the system disclosed herein these possible false operations are prevented by the use of sources of alternating potential in the circuits of the line tube.

Referring to the drawings, the subscribers line I60 appears at the central ofiice in the terminal bank of the line-finder switch F and is provided with a line circuit comprising a line tube IBI, preferably of the gas-filled tube, and a cut-off relay I02. The control electrode B3 of the tube is connected through a high resistance IE4 through normal contacts of the relay I02 and a coupling resistance I05 to the ring conductor IE6 ofthe subscribers line. The tip conductor It'd of the line is connected through a coupling resistance I08 to ground through a back contact of the cut-off relay I92. The negative pole of the central office battery IDS is connected through the left winding and normal contacts of the cutoff relay to the junction point between resistances I04 and I05. The left winding of relay I532 serves as a resistor for conducting the currents which flow over the line from battery I99 when the line loop is closed and when leakage paths are present on the line. The battery I II is normally connected across the control gap of the tube, formed by the control electrode I03 and the cathode I I I, through the left winding and normal contacts of the cut-off relay I02. The voltage of the battery I II], which may be of the order of 55 volts, is insufficient either to ionize the control gap or to sustain the ionization thereof. The main discharge circuit of the tube comprises a section or branch connected to the anode I I2 and includes the group start relay H3 and a secondary winding H4 of the transformer. The other section or branch of the main discharge circuit is connected to the cathode III and includes the secondary winding II5 of the'transformer and batteries H0 and I89 in series. The primary winding II6 of the transformer is connected to a suitable source III of alternating current.

It will be noted that the main discharge circuit of the tube, including the gap formed by the anode I 52 and cathode I I I, is not provided with any relay or switch contacts for quenching the discharge and releasing the group start relay I I3. Accordingly the source H1 is coupled to the anode-cathode circuit by the transformer to quench the anode gap once per cycle, so that the tube will cease to hold relay II3 as soon as the control gap is permanently deionized. To this end the secondary windings IM and H5 are so connected in the anode and cathode branches of the circuit that they aid each other and cumulatively oppose the batteries I09 and III) sufficiently during alternate half cycles of the source I I! to quench the anode discharge. During these periodic interruptions of the current flow in the anode-cathode circuit the relay II3 retains its armatures attracted by reason of its sluggish characteristic, and the relay only releases after the anode discharge has been permanently quenched. The alternating voltage produced by the secondary winding I I4 is considerably greater than that produced by the winding H5. These windings may produce voltages of the order of 40 volts and 5 volts, respectively, and the sum of these two when subtracted from the total voltage of the batteries I69 and Ill), which may have the values of 48 and 55 volts, respectively, reduces the anode-cathode voltage below the sustaining value.

The voltage of the secondary winding II5 0pposes at each half cycle the voltage produced across the control gap 33-4 II by the batteries I09 and H0. But this opposing voltage is comparatively small and does not reduce the control gap voltage below the sustaining value as long as the subscriber has his receiver off the hook and the cut-off relay is in its normal position. When the cut-off relay operates in thenormal manner upon the seizure of the line by the finder, the control gap circuit is opened and the tube ceases to discharge. Similarly when the subscriber replaces his receiver at the end of a call the voltage across the control gap is reduced, provided the line leakage is not excessive, below the sustaining voltage, and the tube ceases to discharge. Under these normal conditions the opposing voltage of the winding H5 is not required to effect the release of the tube. If, however, the line leakage is excessive, the tube may be held in a locked condition, since the additional voltage applied to the control gap may be sufiicient to raise the total voltage to a value above the sustaining voltage of the gap even though the subscribers loop is open. In such a case the voltage supplied by the winding I I becomes effective and opposes the voltage across the control gap at each alternate half cycle, reducing said voltage below the valu suflicient to sustain ionization. Since the opposing voltage of the winding H5 is in phase with the voltage of the other winding II4, the control gap i quenched simultaneously with the quenching of the main gap, and on the next half cycle of the source II1 neither of the gaps reionize because the voltages across them are insufiicient to refire the tube.

Excepting the testing and other controlling circuits, the line finder F may be of the usual step-by-step type having vertical and rotary motions for selecting the calling subscribers lines. The testing functions, whereby the finder discriminates between idle lines, busy lines and calling lines are performed by the discharge tubes I I8 and II9 operating in conjunction with the differential test relay I and also in cooperation with the other circuits of the finder switch. The control electrodes I2! and I22 of these tubes are connected to the test brush I23 of the switch. The cathode I24 of tube I i8 is connected through the transformer winding I25 to ground through batteries I26 and I21, the voltages of which may be 20 and 80 volts. respectively. The cathode I28 of tube H9, however, extends to ground through the transformer winding I29 and battery I 21 only. Thus. the normal potential of the cathode I24 is 100 volts whereas that of the cathode I 28 is only 80 volts. The anode I3i is connected through the right winding of the dif ferential relay I29 and through the transformer winding I33 directly to ground, whereas the anode I3! is connected through the left winding of the relay and the transformer winding I32 to the positive pole of the 20-volt battery I 34. The sum of the voltages in the windings I33 and I25, induced therein by the primary winding i35 of the transformer, which is energized by the source I33, is sufiicient to quench the main gap discharge of the tube I I8, and the voltage of winding I25 simultaneously opposes the voltage across the control gap to insure quenching of the tube in case the control gap voltage rises to L excessive value for any reason. Similarly the voltages of windings I32 and I29 cause the de-- ionization of the tube II9. As will be explained more fully hereinafter, both of the tubes H3 and II9 fire when a busy line is tested, and the relay I20, being differential, remains inert; neither tube fires when an idle non-calling line is tested: and when a calling line is tested only tube I I8 operates, whereupon the test relay I25 attracts its armatures and stops the finder switch. A

high resistance I13, connected between conductor I54 and the point intermediate batteries I25 and I21. serves to prevent false operation of tubes H8 and H9 due to surges or spurious potentials on control electrodes.

The line finder F is connected to the usual first selector switch S which has access through succeeding switches, including selector S- I and connector C, to the called subscribers lines. A

second connector switch C-I, having access .to

the subscribers line I20 is also illustrated to show the manner in which the line circuit is connected thereto.

type, although it should be understood that the invention is not limited to any particular type of system nor to the use of any particular type of switch.

The operation of the system will now be de scribed in detail. Assume for this purpose that the subscriber of line I00 wishes to converse with the subscriber of line I10. When the calling subscriber removes his telephone, a circuit is closed from the negative pole of battery I09 through the left winding and normal contacts of cut-01f relay I02, resistance I05, conductor I96 thence through the loop of the calling line to conductor I01, resistance I 08 to ground through the normal contact of relay I02. Relay I02 is marginal and does not respond to the current flowing from battery I09 through its left winding as long as resistance I05 is in series with it. But the voltage produced by the current flowing through the winding of relay I02 is applied in series with the voltage of battery IIO across the control gap I03-III of the tube. The control gap ionizes, and on each aiding alternation of the source II 1 thereafter current flows in the main discharge circuit, which may be traced from the negative pole of battery I I 0 through the winding II5, cathod III, anode I I2, winding of relay I I3, transformer winding II 4 to the grounded positive pole of battery I09. Relay H3 operates and applies ground potential to the commutator segment I31 for the level of terminals in the finder switch containing the line I00. Relay H3 also places ground on the start conductor I38 which extends through the normal contacts of relay I39, contact springs I40, which are actuated on the tenth rotary step of the switch, winding of slow-release relay I4I to battery. Relay I4I operates and places ground on the sleeve conductor I42. Relay I4I also closes an energizing circuit for the stepping relay I43. This circuit may be traced from ground through the contact of relay I 4|, conductor I44, normal contacts of vertical stepping magnet I45, normal contacts of the rotary stepping magnet I41, left winding of the relay I43 to battery through the normal contacts of relay I39. Relay I43 controls the vertical stepping action of the switch.

The finder switch is stepped in its vertical movement by the magnet I45 which energizes as soon as relay I43 operates. The energizing circuit of the magnet may be traced from battery through the normal contacts of relay I conductor I48, winding of magnet I45, normal con tacts of relay I 49, closed contacts of relay I43 to the grounded conductor I44. At each energization of the magnet I the switch is advanced one step, and the circuit of relay I43 is opened. Each time the circuit of relay I43 is opened, the relay releases the magnet I45, which in turn recloses the circuit of the relay. This action continues until the commutator brush I engages the grounded segment I31, at which time a circuit is extended from the grounded brush I53 through the left winding of relay I49, normal contacts of magnet I41, left winding of relay I43 to battery. Relay I43 remains energized in this circuit and holds the magnet I45 in an energized condition to prevent further stepping of the switch. Relay I49, being slow to operate, attracts its armatures after an interval following the closure of the above-traced circuit through the winding of the relay. Relay I49 in operating locks in a circuit traceable from battery through the contacts of relay I39, conductor E48, winding of vertical magnet I45, conductor I5I, right winding and contact of relay I49 to the grounded conductor I44. Relay I491ocks, but magnet I45 releases in series with the high resistance winding of the relay,

Relay I49 also initiates the rotary stepping movement of the switch. This movement is performed by the rotary magnet I41 which now energizes in a circuit from battery through its winding, front contact of relay I49, front contact of relay I43 to the grounded conductor I44. Magnet I41 advances the brushes of the switch to the first set of terminals in the selected level and also opens the circuit of the stepping relay I43. Relay I43 in turn opens the circuit of the magnet, and the latter releases. On the first rotary step of the switch the commutator brush I50 disengages the grounded segment and opens the circuit in which relay I49 was originally operated and relay I43 was held at the completion of the vertical movement. Relay I43 now reoperates in the circuit previously traced through its left winding, normal contacts of magnet I41 and the normal contacts of magnet I45 to the grounded conductor I44. The reoperation of relay I43 causes the reoperation of magnet I41, and the switch takes a second step. This stepping action continues until the test wiper I 23 encounters the test terminal of the calling line I00.

As the finder switch hunts in the selected level its brushes may encounter both idle non-calling lines and busy lines before reaching the calling line. If a line, such as the line I00, is in an idle non-calling condition, the full negative potential (48 volts) of the central office battery I09 will be present on the test terminal I52. This potential is applied to the terminal I52 over the path extending through the left winding of the cutoff relay I92 and sleeve conductor I53. Assuming no leakage currents are flowing over the subscribers line, the full potential of the battery W9 is present on terminal I52 since the loop of the line is open at the substation and no current is flowing over the line through the winding of the cut-off relay. When, therefore, the test brush I23 of the hunting finder engages the terminal I52 of an idle non-calling line, a negative potential of 48 volts is applied over test conductor I54 and through the resistances I55 and I55 to the control electrodes I2I and I22, respectively,

of the test tubes H8 and H9. The resultant voltage of the battery I09 and batteries I25 and I21, thus applied to the control gap of the tube I I8 is insufiicient, even when aided by the voltage produced in the transformer winding I25, to ionize the control gap. Hence no current fiows in the anode circuit of the tube, which includes the right winding of relay I20. Also the voltage resulting from batteries I09 and I21, applied to the control gap of the tube H9, is insufficient to ionize the tube, and no current flows through the left winding of relay I20. Relay I20 remains deenergized, and the finder passes over the terminals of the idle line thus encountered.

When a subscribers line is; in a busy condition, the test terminal thereof, such as the terminal I52 of the line I00, is identified by the presence of a ground potential thereon. This potential may be supplied to the terminal I 52 from a linefinder switch which previously selected the line over one of its multiple appearances; or the ground may be supplied thereto from a connector switch C-I over the sleeve conductor I51 through the front contacts of the cut-off relay I02 and. conductor I53. In either case this ground potential is extended over the brush I23 I21 aided by the winding I25 is supplied to the control gap of tube I I8 and the potential of battery I21 aided by the winding I29 is applied to the control gap of tube II9. Both tubes ionize, whereupon current flows through both windings of the relay I20, the circuit for the right winding being traceable from the negative pole of battery I26, transformer winding I25, cathode I24, anode 139, right winding of the relay, transformer 533 to ground, and the circuit for the other winding being traceable from the negative pole of battery I21, winding I29, cathode I28, anode I3I, left winding of the relay, winding I32 to the positive pole of battery I34. The voltage of battery I34 is substantially equal to that of battery I26; therefore, substantially equal currents flow through the two windings of the relay I20. Since this relay is differential and also marginal, the flux resulting from the current flowing through these windings is not suflicient to cause the attraction of the armature. Therefore, the finder switch passes the busy line and continues to search for a line having a calling condition thereon.

If, as has already been assumed, the subscribers line I 00 is in a calling condition, the test terminal I52 is characterized by the presence of a negative potential of some intermediate value, such as 25 volts. Since the line is calling and the cut-off relay I02 is still in its normal condition, current flows from the battery I09 through the left winding and normal contacts of the relay, resistance I05 over the loop of the line as previously described. This current flowing through the Winding of the cut-off relay reduces the normal negative potential on the terminal I52 to the intermediate value. When the test brush I23 encounters this terminal, the resultant voltage applied to the control gap of the tube I I8 causes the tube to ionize, whereas the voltage applied to the tube I I9 is insufficient to ionize it. Therefore, current flows through one winding of the relay I20 alone, and the relay attracts its armature.

When relay I20 operates, a circuit is closed from battery through its contacts over conductor I58, through the left winding of relay I59, normal contacts of relay I39, right winding of relay I43 to ground at the front contacts of relay I4I. Relay I59 operates in this circuit, and relay I43 remains energized to prevent any further stepping movement of the switch. Relay I59 locks in a circuit from battery through its right winding and front contacts, through the closed contacts of the magnet I41 to ground at the front contacts of relay I4I. Relay I59 operates relay I39 in a circuit from battery through the right winding of relay I39, front contacts of relay I59 to ground at the off-normal springs I60, which shifted when the switch took its first vertical step. With relay I39 operated a second locking circuit for relay I59 may now be traced from battery through the right winding and front contact of relay I59, front contacts of relay I39 to ground at the front contacts of relay I4I. Relay I39 causes the release of relays I43 and I49. When rela I59 operates it extends the calling subscribers line over the tip and ring conductors through to the impulse relay associated with the first selector switch S. The operation of the impulse relay causes ground potential to be applied to the holding conductor I42 to hold the linefinder switch in its operated condition after the release of relay I4I as explained below. Relay I20 in operating also completes a circuit from ground over its contacts, conductor I54, brush I23, terminal I52, conductor I53 through the left winding of the cut-01f relay I02 to battery I39. Now the resistance I is not in series with the left winding of relay I02, so relay I02 operates and locks in a circuit from battery me, right winding and front contacts of relay and thence to ground as above traced. Relay I82 in operating disconnects the control electrode I03 of the tube IIII from the calling line, whereupon the control gap of the tube becomes deionized. On the next opposing alternation of the source II! the voltage produced by windings [I4 and H5 opposes the batteries ltd and H9 to quench the discharge in the main gap of the tube. After an interval the group start relay I I3 releases, disconnecting ground potential from the commutator segment of the line finder and opening the original energizing circuit of relay Ml. Relay I4I is slow to release and retainsits armatures until the calling line is extended through to the selector switch S in the manner above described. When relay I20 applies ground potential to the conductor I54, the tube I I9 also fires, causing the other winding of relay 12a to energize and release the armature. Relay E59, however, does not release since it is now locked in a circuit from battery through its right winding and contacts, front contacts of relay I39 over conductor M2 to ground applied at the selector switch S. This ground potential on the conductor 52 is also extended over the front contacts of relay I39 and the front contacts of relay I53 to conductor I54 and the multiples of the calling line test terminals to hold the line in a busy condition and to maintain the ionization of tubes H8 and H9.

The calling subscriber manipulates his dial to operate the selector switches S and S% and the connector switch C to complete the connection to the called subscribers line Ila. When the called party answers, relay I'll operates and reverses the direction of current flowing back over the connection and over the calling subscriber's line. The circuit for supplying current to the calling line may now be traced from battery through the upper winding of relay I'E2, upper front contacts of relay III, thence over the tip side of the connection through the brush SI of the line finder over the tip conductor It? of the calling line and returning over the ringing conductor H35 through the finder brush IE2 thence over the ring side of the connection through the lower front contacts of relay Ill and the lower winding of relay I72 to ground. Current flowing over the calling line from this source charges the capacity of the calling line in the direction corresponding to the application of the positive pole of battery to the ring conductor me. This direction, it will be noted, is opposite the direction in which the subscribers line is normally charged by the central oifice battery I89 when connected to the line during the idle condition thereof.

When the subscribers have finished the conversation, they replace their receivers, and relays Ill and I72 release. In the well-known manner the holding ground potential is removed from conductor I42, and relay I59 releases. A circuit is now closed from batter through the release magnet I53, off-normal contacts I6 3, normal contacts of relay I d9, normal contacts of relay I59, normal contacts of relay Il to ground at the off-normal contacts I53. The magnet I63 releases the finder switch to its normal position, and the springs I555 are shifted to open the holding circuit of relay I39 and the circuit of the release magnet I63. The removal of ground. potential from the conductor I54 also causes the extinguishment of the tubes H8 and 9.

When ground potential is removed from test brush I23 at the time the line finder releases, the cut-off relay I62: releases and reconnects the negative pole of battery IE9 through resistance. E5 to the ring conductor I96 of the subscribers line. Since this side of the line is charged with positive potential, the reconnection of the negative pole of battery thereto causes a substantial current to flow from the battery I I39 over the line to recharge it in the opposite direction. This current flowing through the right winding of relay I62 produces suificient voltage across the, control gap of the tube IOI to cause it to ionize. Thereupon current flows in the main anode circuit of the tube through the winding of relay H3. As soon as the line charging current diminishes, the voltage across the control gap falls below the sustaining value, provided it is free from leakage paths. However, if we assume that the line has a leakage path, represented by the resistanc element I64, the current flowing from the battery I09 through thev left winding of relay i212 and thence over the line to ground through the resistance I54 may produce a voltage which, when added to the voltage of the battery I II), is suflicient to sustain ionization in the control gap of the tube after the line charging current ceases to flow. This continued ionization of the tube would cause the operation of the group start relay i I3 and the line-finder switch were it not for the opposing alternating voltages applied to the tube circuit by the transformer windings I M. and H5. But the alternating voltage produced by the transformer winding H5 is in opposition to the voltage that tends to maintain the control gap ionized and is sufficient to overcome the effect of the leakage current and quench the discharge of said gap. This quenching of the con-1 trol gap occurs simultaneously with the quenching of the main gap to insure that the tube will not refire when the alternating voltage reverses its polarity.

The abandonment of a call by the subscriber is another condition under which it is necessary to quench the tube IElI to prevent false operation. Assume that the subscriber initiates a call as above described; whereupon the tube ionizes, the group start relay II3 energizes, and a finder F is set in motion to hunt for the calling line. If the subscriber decides to abandon the call and replaces his receiver on the switchhook while the finder is still hunting the calling line, the potential on the terminal I52 is immediately raised from the intermediate value of 25 volts to the full value of 48 volts present on the terminals of idle non-calling lines. In the meantime, however, the control gap and the main gap of the tube have ionized, and relay I I3 is operated. If the leakage resistance I6 1 on the line is sufiiciently low, the leakage current flowing from battery I89 increases the voltage across the control gap to a value which exceeds the sustaining voltage. Hence the tube will, remain ionized, and the finder will pass by the line and continue to search for a calling line which does not exist. This false operation, however, is prevented as above explained by the opposing alternating voltages set up by the transformer windings H4 and H5, and the tube I5I is restored to its normal condition, and the group start relay H3 releases to discontinue the hunting movement of the finder switch.

Fig. 3 shows an improvement in the line-finder test circuit whereby the discharge tubes, such as the tubes H8 and H9 of Fig. 2, are deionized during the conversational period. The advantage of this arrangement is a substantial increase in the life of the tubes.

To facilitate an understanding of the test circuit shown in Fig. 3 the relays, tubes, and other elements are given reference characters which differ only in the first digit from those assigned to the corresponding elements in Figs. 1 and 2. The test conductor 354 from the test brush I23 of the line finder F extends through the armature and normal contact of relay 339 and through resistances 355 and 358 in parallel to the control electrodes of the test tubes 3l8 and 3l9. The anode and cathode elements of these tubes are connected through the windings of relay 320 and through the windings of the transformer, which is excited by the alternating current source 336, to battery in the manner disclosed in Fig. 2.

When the line finder encounters a calling line, the potential on the test conductor 354 causes tube 3l8 alone to ionize. Current now fiows through the right Winding of relay 329, and the relay operates. Relay 320 closes a circuit from battery through its front contact, conductor 358 through the right winding of the stepping relay 343 thence to ground as shown in Fig. 1. This circuit holds the relay 343 to prevent further stepping of the finder switch. Relay 320 also closes a circuit from ground through its front contacts, conductor 359, through the winding of relay 359 to battery. Relay 359 operates and locks through the front contact of the rotary magnet 341. Relay 359 operates relay 339, and these relays lock as previously described. When relay 339 operates, the test and holding conductor 354 is disconnected from the tubes 3l8 and 319 and is extended to the corresponding conductor 3 32 leading to the selector switch. Tube 3l8 deionizes, relay 32D releases, and the of potential and means for applying the same to quench the discharge of said control gap, and a second source of potential and means for applying the same in series aiding relation with said first source to quench the discharge of said main gap.

3. The combination with a telephone line of a discharge device having a main gap and a control gap, circuit means including said line for ionizing said control gap, means for causing a discharge across said main gap in response to the ionization of said control gap, a source of alternating potential serving to oppose the continued ionization of said control gap, a second source of alternating potential, and means for applying said sources in series aiding relation to quench the discharge of said main gap.

4. The combination with a telephone line of a discharge device having a main gap and a control gap, circuit means including said line-for ionizing said control gap, means for causing a discharge across said main gap in response to the ionization of said control gap, a source of tubes remain deionized during the conversational period.

It will be understood that the invention is not limited to the specific embodiments chosen to illustrate it herein. Numerous variations and changes may be made in the circuits and devices illustrated. For example, the tube structures may vary, and difierent values of the potential sources may be used. Also different means may be employed for supplying the alternating potentials used in the anode and cathode circuits of the tubes.

What is claimed is:

1. The combination with a line of a discharge device having a main gap and a control gap, circuit means controlled over said line for causing a discharge across said control gap, means for causing a discharge across said main gap in response to the discharge of said control gap, a source of potential acting to quench the discharge of said control gap, and a second source of potential connected in serial relation with said first source to quench the discharge of said main gap.

2. The combination with a line of a discharge tube having a main gap and a control gap, circuit means including said line for causing a discharge across said control gap, means for causing a discharge across said main gap in response to the discharge of said control gap, a. source alternating potential acting on each cycle to quench the ionization of said control gap, a second source of alternating potential acting cumulatively with said first source to quench the discharge of said main gap concurrently with the quenching of said control gap.

5. The combination with a telephone line of a discharge tube having a cathode, a control electrode forming a control gap with said cathode, an anode forming a main gap with said cathode, circuit means including said line for causing the ionization of said control gap, an operating circuit including said main gap, means responsive to the ionization of said control gap for causing discharge current to flow across said main gap and in said operating circuit, a source of alternating potential connected in circuit with said cathode for quenching the ionization of said control gap, and a second source of alternating potential connected to said anode and in phase with said first source, said sources of potential.

acting in series relation for the purpose of quenching the discharge across said main gap.

6. The combination with a subscribers line of a discharge tube having a cathode, a control electrode forming a control gap with said cathode, an anode forming a main gap with said cathode, an operating circuit including said main gap, said operating circuit having one section connected to the anode and another section connected to said cathode, means controlled over the subscribers line for causing the ionization of said control gap, a source in said operating circuit for causing current to flow therein and across said main gap in response to the ionization of said control gap, a source of potential in said cathode section for acting to quench the ionization of said control gap, and a second source of current in said anode branch coacting with the source in said cathode branch to quench the discharge of said main gap simultaneously with the quenching of said control gap.

7. The combination with a telephone line of a source of current, a discharge device having a control electrode, resistance means for coupling said control electrode to said line and to said source of current, a cathode for said device forming a control gap with said control electrode, an anode for said device forming a main gap with said cathode, means under the control of said line for closing a circuit including said source of current to increase the voltage across said control gap above the ionizing value, means responsive to the ionization of said control gap for causing a discharge across said main gap, a source of alternating potential for opposing the ionizing voltage across said control gap, and a second source of alternating potential connected in series circuit relation with said last-mentioned source for quenching the discharge across said main gap.

8. In a telephone system, a telephone line, means for closing said line, a space discharge device for said line having a control gap and a main gap, a source of current connected to said line, a resistor in series with said source for conducting from said source the current flowing over said line as a result of the closure thereof and also serving to conduct the leakage currents flowing over said line, the current flowing through said resistor as a result of line closure serving to increase the voltage across said control gap above the ionizing value, means for causing current to flow in said main gap in response to ionization of the control gap, a source of potential opposing the voltage across the control gap caused by said leakage currents to reduce said control gap voltage below the sustaining value when said line is opened, and a second source of potential for quenching the flow of current in said main gap when the voltage across said control gap is reduced below the sustaining value.

9. In a telephone system, a subscribers line, means controlled by the subscriber for closing said line to initiate a call, a space discharge tube having a control electrode and a cathode forming a control discharge gap, an anode forming with said cathode a main discharge gap, a source of current, a resistor for coupling said source to the line and to said control electrode, said resistor serving to conduct the current flowing from said source over said line as a result of line closure and to conduct the leakage currents flowing over said line from said source, the current flowing through said resistor as a result of line closure serving to increase the voltage across said control gap above the ionizing value, means for causing current to flow in said main gap in response to the ionization of the control gap, a source of alternating potential connected to said cathode for nullifying the voltage produced across the control gap as a result of the leakage currents flowing through said resistor, and a second source of alternating potential connected to said anode and acting in phase with said lastmentioned source to quench the discharge across said main gap.

10. The combination with a line of a discharge tube having a main gap and a control gap, circuit means including said line for causing a discharge across said control gap, means for causing a discharge across said main gap in response to the discharge of said control gap, a transformer having a winding for applying a voltage in opposition to the discharge of said control gap, a second winding on said transformer for applying a voltage in series aiding relation to the voltage of said first winding for opposing the discharge of said main gap, and means for energizing said transformer windings.

11. The combination with a discharge tube having a cathode, acontrol electrode forming a control gap with said cathode, an anode forming a main gap with caid cathode, circuit means for causing the ionization of said control gap, an operating circuit including said main gap, means responsive to the ionization of said control gap for causing discharge current to flow across said main gap and in said operating circuit, a transformer having a winding in circuit with said cathode for producing a voltage quenching the ionization of said control gap, a second winding on said transformer connected to said anode for producing a voltage in phase with the voltage of said first winding for the purpose of quenching the discharge across said main gap, and means for energizing said transformer windings.

12. In combination, telephone'lines, a switch having access to said lines, two discharge tubes having their control electrodes connected to the test circuit of said switch for testing the condition of said lines, means for operating said tubes in diflerent combinations according to the condition of the line tested, and a test relay having a plurality of windings energized in difierent combinations in response to the operation of said tubes.

13. In combination, telephone lines, a finder switch having brushes for making connections with said lines, two discharge tubes having their control elements connected to the test brush of said switch for testing the condition of said lines, means for ionizing said tubes in different com binations according to the condition of the line, a differential test relay having a winding in circuit with one of said tubes and a second winding in circuit with the other tube, said windings being energized in different combinations in response to the ionization of said tubes, and means controlled by said relay for stopping said finder switch when the line tested has a particular condition thereon.

14. In a telephone system, a subscribers line subject to the source of current is connected thereto, means controlled by the subscriber for closing the loop of the line to initiate a call, a space discharge tube having a control electrode and a cathode forming a control discharge gap, an anode forming with said cathode a main discharge gap, a source of current, a resistor for coupling said source to the line and to said control electrode, said resistor serving to conduct the current flowing from said source over said line as a result of line loop closure and to conduct leakage currents flowing over said line from said source, the current flowing through said resistor as a result of line loop closure serving to increase the voltage across said control gap above the ionizing value, the current flowing in said resistor'as a result of the leakage on said line acting to increase the voltage across said control gap above the sustaining value, means for causing current to flow in said main gap in response to the ionization of the control gap, a source of alternating potential connected to said cathode and of a magnitude suflicient to overcome the voltage produced by the flow of leakage currents and to reduce the voltage across the control gap below the sustaining value when the line loop is open, and a second source of alternating potential connected to said anode and acting in phase with said last-mentioned source to quench the discharge across said main gap.

MASON A. LOGAN.

flow of leakage currents when a 

